1. Field of the Invention
This invention relates to a method of completing a well utilizing frac packing and gravel packing.
More particularly, this invention relates to a method for placing a gravel pack screen in a wellbore, pumping sand slurry at a high enough rate and pressure to fracture an earthen formation surrounding the wellbore, and after achieving a desired fracture length and width, utilize pressure and rate in a circulating position to test for a sufficient gravel filling of a screen/casing annulus. The method is performed without the use of a crossover service tool, thus abating the problems associated with abrasive wear on such tools caused by impingement of sand particles contained in a gravel slurry. Eliminating the problems associated with erosion, having the ability to test gravel pack screen assembly sand coverage, and pump additional gravel if needed improves reliability of a frac pack/gravel pack process.
2. Description of the Related Art
Subsequent to the drilling of a subterranean oil or gas well, casing is typically set and perforated in conventional fashion. Unconsolidated formations, particularly those containing loose sands and sandstone strata, present problems in well production due to the migration of loose sands and degraded sandstone into the wellbore. A long-standing dilemma in the petroleum industry is the production of these unconsolidated materials in conjunction with the hydrocarbon production. Erosion of production equipment, well plugging, and reduced production levels or loss of the well are some consequences of this problem if left unaddressed.
One method of controlling the production of unconsolidated materials is gravel packing. Gravel packing is essentially a technique for building a two-stage filter downhole. The filter consists of gravel pack sand and a screen or liner. The gravel pack sand is sized according to the particle size distribution of the unconsolidated materials. One method of selecting gravel pack size is discussed in U.S. Pat. No. 3,670,817, entitled “Method of Gravel-Packing A Production Well Borehole,”incorporated herein by reference. The screen or liner has openings that are sized to retain the gravel pack sand. Thus the gravel pack sand retains the unconsolidated formation materials and the screen or liner retains the gravel pack sand. The gravel pack sand and the screen or liner act together to reduce or eliminate the production of the unconsolidated formation materials with the oil or gas production.
One problem associated with gravel packing which hinders hydrocarbon production is the trapping of formation damage in the reservoir. Formation damage can come from numerous sources. Drilling muds used during well construction can invade the formation blocking pore spaces and swelling clay materials in the reservoir. The perforating charges used to penetrate the casing and cement can crush the formation and generate significant quantities of fines (very small particle sized materials). Also, many times high viscosity polymer gels are used to control fluid losses to the reservoir after perforating. These polymer gels can also yield significant restrictions to production. The industry has resorted to fracturing the unconsolidated formations in an effort to bypass any damage which may be trapped by the gravel pack. The term “Frac Pack” is used to describe hydraulically fracturing an unconsolidated formation and at the same time building the two-stage filter of the gravel pack.
In the course of preparing a subterranean well for the installation of a Frac Pack, the well is perforated and the Frac Pack assembly run into the well on a workstring. The Frac Pack assembly generally consists of a well packer containing a crossover service tool and one or more screens or liners. The crossover service tool allows fluid flow in the workstring to exit in an annulus below the packer after the packer is set. Generally the crossover service tool has at least two ports, a primary port ans a circulating port. The primary port allows sand slurry to be pumped down the workstring and into the screen/casing annulus. The circulating port allows gravel pack sand slurry carrier fluid to flow into the workstring/casing annulus above the packer. These two ports allow the crossover service tool to have four fluid flow positions. One of the positions is the squeeze position, where the primary port is below the packer and the circulating port is sealed in the packer bore. This position forces all fluid flow into the formation. Another position is the lower circulating position, where the primary port is below the packer and the circulating port is open above the packer. This position allows fluid to flow into the annulus below the packer, through openings in the gravel pack screen or liner, through a return in the crossover service tool bypass area, out of the circulating port, and into the workstring/casing annulus above the packer. Another position is the upper circulating position, where the primary port is positioned above the packer. Fluid can be pumped down the workstring directly into the workstring/casing annulus above the packer. The last position is the reverse position, where either the circulating port and/or primary port is above the packer and fluid is pumped into the worksheet/casing annulus and into the workstring.
At the well site, the casing is perforated across the productions zone to allow production fluids to enter the wellbore. Primary sand screens are installed in the flow path opposite the perforations in the casing. Packers are set above and below the sand screens to seal off the annular region where production fluids are permitted to flow into the tubing. The crossover service tool is then release from the packer and positioned to perform either a gravel pack or Frac Pack.
In a typical gravel pack installation, the gravel pack sand slurry is generally pumped at low rates, about 2 to about 4 barrels per minute. The crossover service tool is placed in the upper circulating position and the gravel pack sand slurry is pumped into the workstring until the gravel pack sand slurry is within about 150 to about 300 feet of the primary port. The crossover service tool is then positioned into the lower circulating position and pumping continued. This allows the sand slurry to travel into the screen/casing annulus. The sand is retained by the screen or liner and the gravel pack carrier fluid travels into the workstring/casing annulus above the packer. When the pump pressure at the surface indicates gravel pack sand has covered the screen or liner the crossover service tool is placed in the squeeze position to force sand slurry into the perforations. The gravel pack carrier fluid leaks off into the formation allowing a tightly packed sand filter to remain in place. When the pump pressure at the surface reaches sand out pressure, usually about 1500 pounds per square inch above the circulating pressure, the pumps are switched from the workstring to the annulus. The annulus is pressurized with completion fluid and the crossover service tool is placed in the reverse position and any excess gravel pack slurry is reversed from the workstring. After waiting a sufficient time (usually about two hours) for the gravel pack sand in the annulus between the casing and screen or liner to settle, the crossover service tool is again placed in the lower circulating position. The workstring is then pressurized to ensure that the screen or liner is completely covered with gravel pack sand. The crossover service tool is pulled from the well and additional zones gravel packed or production seals and production tubing run into the well.
In a Frac Pack installation, the Frac Pack sand slurry is generally pumped at high rates, about 6 to about 12 barrels per minute. The crossover service tool is placed in the upper circulating position and the Frac Pack sand slurry pumped into the workstring until the gravel pack sand slurry is within about 150 and about 300 feet of the primary port. The crossover service tool is then positioned into the squeeze position and pumping continued. This allows the sand slurry to travel into screen/casing annulus, through the perforations and into the formation. Pumping continues until a tip screen out is achieved. At this point, the surface area of the fracture is dehydrated and the Frac Pack sand slurry in the fracture creates a wide fracture width and bypassing any potential formation damage. When the pump pressure at the surface indicates no additional sand can be placed in the fracture, the crossover service tool is placed in the lower circulating position to dehydrate Frac Pack sand against the screen or liner. The sand is retained by the screen or liner and the Frac Pack carrier fluid travels into the workstring/casing annulus above the packer. When the pump pressure at the surface reaches sand out pressure, usually about 1500 pounds per square inch above the circulating pressure, the pumps are switched from the workstring to the annulus. The annulus is pressurized with completion fluid and the crossover service tool is placed in the reverse position and any excess Frac Pack slurry is reversed from the workstring. After waiting a sufficient time (usually about two hours) for the Frac Pack sand in the annulus between the casing and screen or liner to settle, the crossover service tool is again placed in the lower circulating position. The workstring is then pressurized to ensure that the screen or liner is completely covered with Frac Pack sand. The crossover service tool is pulled from the well and production seals and production tubing run into the well. Typical gravel packing and frac packing methods are shown in the prior art, examples include U.S. Pat. No. 3,987,854, entitled “Gravel Packing Apparatus and Method;” U.S. Pat. No. 4,606,408, entitled “Method and Apparatus For Gravel Packing A Well;” and U.S. Pat. No. 4,627,488, entitled “Isolation Gravel Packer,” incorporated herein by reference. Such typical and other prior art gravel packing assemblies contain a form of the crossover service tool.
There are other methods of gravel packing and frac packing which do not utilize a crossover service tool. One method is a bottom up treatment described in U.S. Pat. No. 2,978,024, entitled “Method of Gravel Packing Well Treatment;” U.S. Pat. No. 3,602,307, entitled, “Apparatus and Method for Gravel Packing Wells;” and U.S. Pat. No. 5,722,490, entitled, “Method of Completing and Hydraulic Fracturing of a Well,” incorporated herein by reference. These ability to establish a circulating position to test the annular integrity of the gravel or frac sand in the screen/casing annulus.
The multiple flow positions obtainable with the crossover service tool makes it the preferred method of installing a gravel pack or Frac Pack completion. It has been observed that the turbulence within the downwardly flowing gravel packing or fracturing sand slurries results in an actual erosion or cutting of the internal wall of the crossover service tool. The erosion can be severe enough to cut entirely through the crossover service tool primary flow port causing communication with the return fluid flow path to the circulating port. This communication between the primary and circulating ports can prevent gravel pack or Frac Pack slurry placement in the screen/casing annulus. In a worst case scenario, the gravel pack or Frac Pack slurry can travel inside the screen or liner. When this problem is encountered, the well must be completely closed in and the workstring, packer, and liner assembly with the crossover service tool must be completely withdrawn from the well and replaced. This results in considerable downtime and extra rig time utilization. The turbulent fluid effects upon gravel packing systems during Frac Pack operations is discussed by the inventor in paper number 22857 of the Society of Petroleum Engineers entitled “Study of Effects Upon Gravel-Pack Systems During Frac/Pack Operations,” (1991).
Another problem that is encountered in Frac Pack operations is the result of the high flow rates required to fracture the formations and discus drag forces of the gravel pack or Frac Pack slurries. The high flow rates make it difficult to anticipate a wellbore screen out. The screens or liners are normally positioned to extend above the perforations. As the perforations become covered with sand, all flow into the formation must first go through the screen or liner assembly. The high flow rates and discus drag forces can cause damage to the screen or liner resulting in an inability to retain the gravel pack or Frac Pack sand. When the well is placed on production the integrity of the two-stage filter is lost and sand may be produced with the hydrocarbons.
Thus, there is a need in the art for a more efficient system for frac packing a well that does not requires use of a crossover service tools, yet allows for installation and testing of a gravel pack assembly even after well fracturing.